Fungus in water?

Hi im just wondering if anyone can tell me wots causing this furry surface on my seedlings?
I have noticed that its everytime when I water them using water from the tub I collect from rainwater
Is it bad?
the seedlings look to be finegrowing still but im worried it might cause problems later on
So if anyone can tell me wot it is I wud be gratefull thanks

 

Are you sure it's only that water? More likely that you're overwatering and it's in the compost so you're providing good growing environment. Shouldn't be harmful

 

Looks like mould, cant be good. I agree with westwales most likely because of over watering and too warm. If you water it with 5% hydrogen peroxide should kill it all off.